PURPOSE: To simplify the constitution of the transmitter eliminating the need for frame synchronism by providing a violation designating pulse generating circuit to a circuit sampling an auxiliary signal.

CONSTITUTION: A transmission section 20 and a receiving section 27 are operated by using a clock pulse 10 as a reference pulse, a main signal 1 inputted to the transmission section 20 is converted into a CMI code train by a CMI converting circuit 21 and processed into random code where (1) and (0) are not consecutive. Further, a periodic auxiliary signal 2 inputted to the transmission section 20 is sampled by a sampling circuit 22, and when a sampling pulse 24 is at (1) level by the violation designating pulse generating circuit 23, a violation designating pulse 25 is applied to the circuit 21 after 2-bit to the time slot. When this pulse 25 has (0) time slot, the pulse is coded in the CMI code rule, and when the pulse has the time slot of (1), the pulse is coded by other code rule and transmitted to the receiving side 27. This signal is applied to a violation detecting circuit 29 and a CMI decoding circuit 28 so as to output a main signal 6 and an output 7 of an auxiliary signal.